Here's how to proceed.
You must create a modification of template
Go to the following links : admin.php?template-modifications/add&type=public
Template : xc_ytb_video_widget_ytb_video
Modification Key : xc_ytb_video_widget_ytb_video_extension
Description : Enlarging youtube video
Find :
HTML:
{{ widget_data($widget) }}
Replace :
To change the width you have to replace the value "350" by what you want
Execution order : Not change
AddOn : Not Change